1. Field of the Invention
The present invention relates to an intermediate relay station and, more particularly, to an intermediate relay station which employs a heterodyne relay system and drops and inserts a frequency-modulated SC signal (service channel signal).
2. Description of the Related Art
A microwave communication system employing transmitting and receiving terminal stations and intermediate relay stations therebetween needs to transmit a service channel signal (hereinafter, referred to as an SC signal), besides a main signal, in order to monitor and control the intermediate relay stations.
An intermediate relay station in a heterodyne-relay microwave communication system relays a main signal as follows: an intermediate relay station converts a microwave received from the preceding station into an intermediate frequency, and then converts the intermediate frequency into a microwave to transmit to the succeeding station.
This system does not regenerate data by converting a main signal into a baseband signal and, therefore, is also called a non-regenerative relay system. To monitor and control the intermediate relay stations, this system needs to drop or insert, at the intermediate relay stations, a monitor/control signal, called an SC signal, that is sent from the preceding station, without resorting to the main signal. Therefore, the intermediate relay stations of this system are provided with means for extracting and adding an SC signal. The transmitting terminal station superposes an SC signal by frequency-modulation onto a microwave carrier which has been PSK-modulated by digital signals of a main signal. When receiving the microwave, each intermediate relay station converts the microwave into an intermediate frequency signal (an IF received signal) in order to extract an SC signal, which is then used for a predetermined purpose. Each intermediate relay station adds a new SC signal (which may include a part of the extracted SC signal at that station) to the microwave by frequency modulation during conversion of the IF received signal into a sending microwave signal. The IF received signal used for the frequency modulation contains a frequency-modulation component corresponding to the SC signal added by the preceding station, as well as a PSK-modulation component corresponding to the main signal. Therefore, if each intermediate relay station directly uses such an IF received signal for conversion into a sending microwave, the frequency modulation (FM) factor of the microwave is increased by the SC signal added by each intermediate relay station. In other words, as the microwave is relayed by the plurality of intermediate relay stations to the receiving terminal station, the FM factor and the frequency deviation of the microwave carrier become greater. Although the intermediate relay stations and the receiving terminal station each have a PLL (phase-locked loop) function for carrier regeneration and data demodulation, the PLL function fails to perform, undesirably if the frequency deviation of the microwave received is excessively increased, thus making carrier regeneration or the like difficult.
In many cases adopting the above mentioned communication system, the intermediate relay stations employ the SD(space diversity) reception system which combines waves received through two radiowave propagation paths, in order to enhance the received signal which has been degraded by variation of the radiowave propagation characteristics, such as fading. Such intermediate relay stations should also be able to optimally combine two separately-received waves, transmit an SC signal having a sufficiently large bandwidth and add no FM factor increment.